Thiol

In organic chemistry, a thiol (/ˈθaɪˌɔːl/, /ˈθaɪˌɒl/)[1] is an organosulfur compound that contains a carbon-bonded sulfhydryl (–C–SH or R–SH) group (where R represents an alkane, alkene, or other carbon-containing group of atoms). Thiols are the sulfur analogue of alcohols (that is, sulfur takes the place of oxygen in the hydroxyl group of an alcohol), and the word is a portmanteau of "thion" + "alcohol," with the first word deriving from Greek θεῖον ("thion") = "sulfur."[note 1] The –SH functional group itself is referred to as either a thiol group or a sulfhydryl group.

Many thiols have strong odors resembling that of garlic. Thiols are used as odorants to assist in the detection of natural gas (which in pure form is odorless), and the "smell of natural gas" is due to the smell of the thiol used as the odorant.

Thiols are often referred to as mercaptans.[3][4] The term mercaptan/mərˈkæptæn/[5] is derived from the Latinmercurium captans (capturing mercury)[6] because the thiolate group bonds very strongly with mercury compounds.[7] Thiols react with mercury to form mercaptides.[8]

Thiols and alcohols have similar molecular structure. The major difference is the size of the chalcogenide, C–S bond lengths being around 180 picometers in length and C-O bond lengths being around 143 picometers in length. The C–S–H angles approach 90°. In the solid or molten liquids, the hydrogen-bonding between individual thiol groups is weak, the main cohesive force being van der Waals interactions between the highly polarizable divalent sulfur centers.

Due to the lesser electronegativity difference between sulfur and hydrogen compared to oxygen and hydrogen, an S–H bond is less polar than the hydroxyl group. Thiols have a lower dipole moment relative to the corresponding alcohol

Many thiols have strong odors resembling that of garlic. The odors of thiols are often strong and repulsive, in particular for those of low molecular weight. The spray of skunks consists mainly of low-molecular-weight thiols and derivatives.[9][10][11][12][13] These compounds are detectable by the human nose at concentrations of only 10 parts per billion.[14] Human sweat contains (R)/(S)-3-methyl-3-sulfanylhexan-1-ol (MSH), detectable at 2 parts per billion and having a fruity, onion-like odor. Women emit significantly more MSH than men.[15] (Methylthio)methanethiol (MeSCH2SH; MTMT) is a strong-smelling volatile thiol, also detectable at parts per billion levels, found in male mouse urine. Lawrence C. Katz and co-workers showed that MTMT functioned as a semiochemical, activating certain mouse olfactory sensory neurons, attracting female mice.[16] Copper has been shown to be required by a specific mouse olfactory receptor, MOR244-3, which is highly responsive to MTMT as well as to various other thiols and related compounds.[17]

Thiols are also responsible for a class of wine faults caused by an unintended reaction between sulfur and yeast and the "skunky" odor of beer that has been exposed to ultraviolet light.

Natural gas distributors were required to add thiols, originally ethanethiol, to natural gas (which is naturally odorless) after the deadly New London School explosion in New London, Texas, in 1937. Many gas distributors were odorizing gas prior to this event. Most gas odorants utilized currently contain mixtures of mercaptans and sulfides, with t-butyl mercaptan as the main odor constituent. In situations where thiols are used in commercial industry, such as liquid petroleum gas tankers and bulk handling systems, an oxidizing catalyst is used to destroy the odor. A copper-based oxidation catalyst neutralizes the volatile thiols and transforms them into inert products.

Thiols show little association by hydrogen bonding, with both water molecules and among themselves. Hence, they have lower boiling points and are less soluble in water and other polar solvents than alcohols of similar molecular weight. For this reason also, thiols and corresponding thioether functional group isomers have similar solubility characteristics and boiling points, whereas the same is not true of alcohols and their corresponding isomeric ethers.

Such reactions are conducted in the presence of acidic catalysts. The other principal route to thiols involves the addition of hydrogen sulfide to alkenes. Such reactions are usually conducted in the presence of an acid catalyst or UV light. Halide displacement, using the suitable organic halide and sodium hydrogen sulfide has also been utilized.[19]

In general, on the typical laboratory scale, the direct reaction of a halogenoalkane with sodium hydrosulfide is inefficient owing to the competing formation of thioethers Instead, alkyl halides are converted to thiols via a S-alkylation of thiourea. This multistep, one-pot process proceeds via the intermediacy of the isothiouronium salt, which is hydrolyzed in a separate step:[20]

CH3CH2Br + SC(NH2)2 → [CH3CH2SC(NH2)2]Br

[CH3CH2SC(NH2)2]Br + NaOH → CH3CH2SH + OC(NH2)2 + NaBr

The thiourea route works well with primary halides, especially activated ones. Secondary and tertiary thiols are less easily prepared. Secondary thiols can be prepared from the ketone via the corresponding dithioketals.[21] A related two-step process involves alkylation of thiosulfate to give the thiosulfonate ("Bunte salt"), followed by hydrolysis. The method is illustrated by one synthesis of thioglycolic acid:

Akin to the chemistry of alcohols, thiols form thioethers, thioacetals, and thioesters, which are analogous to ethers, acetals, and esters respectively. Thiols and alcohols are also very different in their reactivity, thiols being more easily oxidized than alcohols. Thiolates are more potent nucleophiles than the corresponding alkoxides.

Relative to the alcohols, thiols are more acidic. The conjugate base of a thiol is called a thiolate. Butanethiol has a pKa of 10.5 vs 15 for butanol. Thiophenol has a pKa of 6 vs 10 for phenol. Thus, thiolates can be obtained from thiols by treatment with alkali metal hydroxides.

With metal ions, thiolates behave as ligands to form transition metal thiolate complexes. The term mercaptan is derived from the Latinmercurium captans (capturing mercury)[6] because the thiolate group bonds so strongly with mercury compounds. According to Hard/Soft Acid/Base (HSAB) theory, sulfur is a relatively soft (polarizable) atom. This explains the tendency of thiols to bind to soft elements/ions such as mercury, lead, or cadmium. The stability of metal thiolates parallels that of the corresponding sulfide minerals.

The catalytic cycle for ribonucleotide reductase, demonstrating the role of thiyl radicals in producing the genetic machinery of life.

Free radicals derived from mercaptans, called thiyl or thiol radical or mercapto radical, are commonly invoked to explain reactions in organic chemistry and biochemistry. They have the formula RS where R is an organic substituent such as alkyl or aryl.[4] They arise from or can be generated by a number of routes, but the principal method is H-atom abstraction from thiols. Another method involves homolysis of organic disulfides.[26] In biology thiyl radicals are responsible for the formation of the deoxyribonucleic acids, building blocks for DNA. This conversion is catalysed by ribonucleotide reductase (see figure).[27] Thiyl intermediates also are produced by the oxidation of glutathione, an antioxidant in biology. Thiyl radicals are also intermediates in the vulcanization process. For example, the vulcanization of polyisoprene results when mercapto radicals couple forming disulfide and polysulfidecrosslinks. Thiyl radicals (sulfur-centred) can transform to carbon-centred radicals viahydrogen atom exchange equilibria. The formation of carbon-centred radicals could lead to protein damage via the formation of C—C bonds or backbone fragmentation.[28]

As the functional group of the amino acidcysteine, the thiol group plays a very important role in biology. When the thiol groups of two cysteine residues (as in monomers or constituent units) are brought near each other in the course of protein folding, an oxidation reaction can generate a cystine unit with a disulfide bond (-S-S-). Disulfide bonds can contribute to a protein's tertiary structure if the cysteines are part of the same peptide chain, or contribute to the quaternary structure of multi-unit proteins by forming fairly strong covalent bonds between different peptide chains. A physical manifestation of cysteine-cystine equilibrium is provided by hair straightening technologies.[29]

Many cofactors (non-protein-based helper molecules) feature thiols. The biosynthesis and degradation of fatty acids and related long-chain hydrocarbons is conducted on a scaffold that anchors the growing chain through a thioester derived from the thiol Coenzyme A. The biosynthesis of methane, the principal hydrocarbon on Earth, arises from the reaction mediated by coenzyme M, 2-mercaptoethyl sulfonic acid. Thiolates, the conjugate bases derived from thiols, form strong complexes with many metal ions, especially those classified as soft. The stability of metal thiolates parallels that of the corresponding sulfide minerals.

^"Mercaptan" (ethyl thiol) was discovered in 1834 by the Danish professor of chemistry William Christopher Zeise (1789–1847). He called it "mercaptan", a contraction of "corpus mercurium captans" (mercury-capturing substance) [p. 88], because it reacted violently with mercury (II) oxide ("deutoxide de mercure") [p. 92]. See: